Answer
$64a^6-25$
Work Step by Step
Using $(a+b)(a-b)=a^2-b^2$ or the product of the sum and difference of like terms, the given expression, $
(8a^3+5)(8a^3-5)
$, is equivalent to
\begin{array}{l}\require{cancel}
(8a^3)^2-(5)^2
\\\\=
64a^6-25
.\end{array}